RF and Microwave transistor chips are best characterized “on wafer”. This allows avoidance of parasitic connection elements, like wire bonds and fringe capacitors, which are associated with packaging the devices in order to mount them in test fixtures. It also allows a much larger number of devices to be tested “in situ” without having to laboriously slice the wafer, mount and wire-bond each individual chip. The “on wafer” testing is at this time the preferred testing method, except for very high power devices, beyond 10 Watt RF power. On-wafer testing is also the exclusive testing method in millimeter-wave frequencies, since device packaging is extremely difficult and the parasitic elements associated with the package (inductance of wire bonds and fringe capacitors of package housings) would falsify the measured data to the point of uselessness.
Only a few manufacturers [1, 2] offer wafer probes capable of reliably testing chips in millimeter-wave frequencies (frequency >40 GHz). The millimeter-wave waveguide probes (FIG. 1) are made, usually, using small coaxial cable sections with diameters of the order of 1 mm (0.04″) or less that end into a “coplanar” structure, where the center conductor of the coaxial cable becomes the center conductor of the coplanar section and the ground mantle of the coaxial cable ends up as the ground plan of the coplanar (FIG. 2b). The simple reason is that this is a practical way for the RF signal to be injected and retrieved from planar micro-chips, where all RF contacts are on the same surface (Ground-Signal-Ground or “GSG” configuration). The coaxial cable is then coupled into a waveguide section and ends up at a waveguide flange (FIG. 1).
Therefore at least three connection points (probe tips) are necessary to establish a GSG (Ground-Signal-Ground) contact (FIG. 2b). Since three probe tips do not necessarily form a straight line and since the probes themselves may be imperfect, due to the microscopic structures (distance between probe tips of the order of 150 micrometers ˜0.006″) and manufacturing tolerances, it often happens that the contact between the probe tips and chip contact plots is uneven and unreliable (FIGS. 2b, 3a).
It is therefore necessary to have a planarization option: in the art of semiconductor device testing the misalignment angle between the chip-plot surface and the surface through the probe tips is called “theta” (Θ, FIG. 2b); the process of aligning the probe tips with the chip-plot surface is called “theta adjustment”.
In the case of coaxial wafer probes the coaxial connector of the wafer probe is connected to the test equipment (network analyzers, signal sources, power meters, etc.) using flexible coaxial cables (FIG. 4). In this case the “theta adjustment” is done using a rotating base of the probe holder and a micrometer screw to adjust the angle of the probe tips. This ensures that the probe tips, when they make contact with the chip plots leave equal marks (FIG. 3b) instead of unequal marks (FIG. 3a) when the plan of the probe (probe tips) is not parallel with the plan of the chip (wafer) plots.
However, in the case of millimeter-wave waveguide probes (FIG. 1) the above mentioned theta alignment mechanism and tool (FIG. 4) cannot be used. This is because the waveguide flange is aligned and solidly screwed on the waveguide section of the auxiliary supporting equipment (waveguide signal sources and receivers and/or impedance tuners) which are bulky and cumbersome to mobile handling and fixing under various angles and cannot rotate freely. In these cases provision must be taken for the waveguide probes to be capable of some rotation so they can be “theta aligned” before firmly fastened to the surrounding support equipment. This can be done but needs a special alignment tool and a special probe flange capturing and support apparatus. These items and alignment method are the purpose of this invention.